Calculators
Type numbers, get answers — every box below updates as you type, shows the formula it used, and works fully offline. All figures are rough planning aids, not precision instruments.
Water storage
How much water to put back for drinking, cooking and hygiene — see Water Storage for containers and rotation.
Waiting for numbers…
Formula: gallons = people × days + (pet lb × days ÷ 128 oz/gal) · liters ≈ gallons × 3.785 · filled weight ≈ gallons × 8.34 lb/gal.
Food calories
Rough planning figures only — real needs vary by person, weather and workload. Pair this with Food Storage Basics.
Waiting for numbers…
Rough planning figures: total kcal = people × days × kcal/day. Suggested macro split — grains/carbs 50% of kcal ÷ 4 g, protein 20% ÷ 4 g, fat 30% ÷ 9 g.
Generator runtime
The honest way to estimate runtime: divide tank size by what your machine actually burns per hour, measured or straight from the manual.
Waiting for numbers…
Formula: hours = fuel gallons ÷ gallons per hour.
| Appliance | Running | Starting surge |
|---|---|---|
| Refrigerator | 600 W | 1,800 W |
| Furnace fan | 800 W | — |
| Lights (LED, whole house) | 100 W | — |
| Phone/power bank charging | 10 W | — |
Bleach disinfection
Drops of plain unscented liquid bleach per batch of water — same numbers as Water Treatment. Stir, wait 30 minutes, sniff for a faint chlorine smell.
Waiting for numbers…
Formula: drops = gallons × 8 (at 6%) or × 6 (at 8.25%), doubled when cloudy · 1 teaspoon ≈ 100 drops.
Bleach has limits
Plain sodium hypochlorite 5–9%, unscented, not expired. Splash-less, thickened, scented, or "color-safe" formulas are not for water treatment, and bleach weakens with age.
Dose helper
Educational only — verify before every dose
This helper mirrors the standard amounts printed on US over-the-counter packaging, for reference when original packaging is lost. It is not medical advice. Verify every dose against the actual product label or a pharmacist/doctor — formulations vary by brand and country. Full tables and rules: Dosing by Weight.
Waiting for numbers…
Method: match weight to the packaging weight bands (identical to Dosing by Weight) — under band minimum means ask a professional, never guess.
Battery bank runtime
Enter capacity directly in watt-hours, or let the amp-hour helper compute it. Efficiency covers inverter losses.
Pack: enter Wh, or Ah with voltage.
Waiting for numbers…
Formula: usable Wh = capacity Wh (or Ah × V) × efficiency ÷ 100 · hours = usable Wh ÷ device watts.
Honesty about cycle life
Lead-acid batteries last far longer if you never drain them past half. Unless the manufacturer says otherwise, plan runtime on 50% of rated capacity — treat a "100 Ah" lead-acid as 50 Ah of real storage. Lithium chemistry tolerates deeper discharge, at higher cost.